Some hemodialytic procedure, especially ultrafiltration or hemofiltration, is known to show a circulatory stability during hemodialysis. Dynamics of fluid exchanges was treated mathematically to see factors which may affect the hemodynamic stability. The circulating blood volume is apparently determined by the balance between the removal rate of fluid at dialyser and the capillary refilling rate in the microcirculation. The latter is described by Starling's law. The solutes are driven by Fick's law. Upon the assumption that the circulatory system is stable as long as the circulating blood volume is above certain critical value, it was shown that not only the removal rate of fluid but also the osmolality change created at the dialyser during the passage of blood will possibly affect the stability. This drop seems, therefore, the one reason of the difference found in the circulatory complication in several methods of hemodialysis and seems compatible with the result showing the importance of plasma osmolality to prevent the complication.
